A polycarbonate is excellent in heat resistance, impact resistance and transparency, so that, in recent years, it has been widely used in many fields.
In a preparation process of the polycarbonate, many investigations have heretofore been done. Among these, a polycarbonate derived from an aromatic dihydroxy compound, for example, 2,2-bis(4-hydroxyphenyl)propane (hereinbelow, also referred to as “bisphenol A”) has been industrially produced by any preparation processes of the interfacial polymerization method or the melt polymerization method.
According to the interfacial polymerization method, the polycarbonate is produced from bisphenol A and phosgene, but poisonous phosgene must be used. Also, there remain the problems that the apparatus is corroded by a chlorine-containing compound such as by-produced hydrogen chloride and sodium chloride, and methylene chloride used as the solvent with a large amount, etc., and that removal of the impurities such as sodium chloride, etc., and remaining methylene chloride which cause effects on the polymer physical property, is difficult.
On the other hand, as a method for preparing a polycarbonate from an aromatic dihydroxy compound and a diaryl carbonate, for example, it has been known a melt polymerization method from long ago in which bisphenol A and diphenyl carbonate are polymerized in a melt state by transesterification, while removing the by-produced aromatic monohydroxy compound. The melt polymerization method has merits that it does not use a solvent, etc., different from the interfacial polymerization method. However, the melt polymerization method has a fundamental problem in that, as the polymerization proceeds, the polymer being produced in the system is rapidly increased in viscosity, so that it becomes difficult to efficiently remove the by-produced aromatic monohydroxy compound from the system, and thus the reaction rate is markedly lowered, making it difficult to increase the polymerization degree.
For solving this problem, various methods of removing the aromatic monohydroxy compound from the high-viscosity polymer have been studied. For example, patent document 1 (JP S50-19600B) discloses a screw polymerizer having a vent portion, and further patent document 2 (JP H2-153923A) discloses a method using a combination of a thin film evaporation apparatus and a horizontal polymerization apparatus.
Further, patent document 3 (U.S. Pat. No. 5,521,275B) discloses a method in which the molecular weight conversion of an aromatic polycarbonate is conducted in the presence of a catalyst under reduced pressure conditions using an extruder having a polymer seal portion and a vent portion.
However, by the methods disclosed in these patent documents, a polycarbonate having a satisfactorily increased molecular weight cannot be obtained. The above-mentioned method using a large amount of a catalyst or polymerization performed under severe conditions such that the polymer being produced is under high shear adversely affects the resin, for example, causes the deterioration of hue of the resin or causes a crosslinking reaction in the resin.
Furthermore, with respect to the melt polymerization method, it has been known that the polymerization degree of a polycarbonate is increased by adding a polymerization promoter to the reaction system. The increase of the molecular weight of polycarbonate achieved in a shortened reaction detention time at a lowered reaction temperature increases the amount of the polycarbonate produced, and further facilitates the design of a simplified, inexpensive reactor.
Patent document 4 (EP 0595608B1) discloses a method in which some diaryl carbonates are reacted upon the molecular weight conversion. However, a significant increase of the molecular weight cannot be achieved in this method. Further, patent document 5 (U.S. Pat. No. 5,696,222B) discloses a method for producing a polycarbonate having an increased polymerization degree by adding a certain type of polymerization promoter, for example, an aryl ester compound of carbonic acid or a dicarboxylic acid, such as bis(2-methoxyphenyl) carbonate, bis(2-ethoxyphenyl) carbonate, bis(2-chlorophenyl) carbonate, bis(2-methoxyphenyl) terephthalate, or bis(2-methoxyphenyl) adipate. Patent document 5 teaches that when an ester compound is used as a polymerization promoter, an ester linkage is introduced, so that a polyester-carbonate copolymer (instead of a homopolymer) is formed, and thus the resultant polymer has poor hydrolytic stability.
Patent document 6 (JP Patent No. 4112979) discloses a method in which some bissalicyl carbonates are reacted for obtaining an aromatic polycarbonate having an increased molecular weight.
Patent document 7 (JP 2008-514754A) discloses a method in which, for example, a polycarbonate oligomer and bissalicyl carbonate are introduced into an extruder to effect a polymerization.
Further, patent document 8 (JP Patent No. 4286914) discloses a method in which the terminal hydroxyl group concentration is increased using an active hydrogen compound (dihydroxy compound), and then the resultant aromatic polycarbonate having an increased terminal hydroxyl group concentration is subjected to coupling using a salicylate derivative.
The method disclosed in the above patent document, in which it is necessary to increase the terminal hydroxyl group of a polycarbonate, however, are disadvantageous not only in that a reaction step for an active hydrogen compound and a reaction step for a salicylate derivative are required to cause the production steps to be cumbersome, but also in that the polycarbonate having many hydroxyl group terminals has poor heat stability, leading to a danger of lowering of the physical properties. Further, the increase of the hydroxyl group amount using an active hydrogen compound induces partially a chain cleavage reaction, causing broadening of the molecular weight distribution. Furthermore, for obtaining a satisfactory reaction rate, it is necessary to use a catalyst in a relatively large amount, and, in such a case, a possibility is considered that the physical properties required for the molding become poor.
Several methods for producing a polycarbonate by adding a diol compound to the reaction system have been proposed. For example, patent document 9 (JP H6-94501B) discloses a method for producing a polymeric polycarbonate by introducing 1,4-cyclohexanediol. In the method disclosed in this patent document, however, both 1,4-cyclohexanediol and an aromatic dihydroxy compound are charged into the polycondensation reaction system in advance, and therefore it is presumed that 1,4-cyclohexanediol is first consumed in a reaction for forming polycarbonate (oligomer formation), and then the aromatic dihydroxy compound is reacted to effect a polymerization. For this reason, the method has a disadvantage in that a relatively long reaction time is required, so that the produced polycarbonate is likely to have poor physical properties for external appearance, such as hue.
Patent document 10 (JP 2009-102536A) has a description of a method for producing a polycarbonate, in which a specific aliphatic diol and an ether diol are copolymerized. However, the polycarbonate disclosed in this patent document has an isosorbide skeleton as a main structure, and hence cannot exhibit excellent impact resistance required for the aromatic polycarbonate.
Further, a method in which a cyclic carbonate compound is added to the reaction system (patent document 11; JP Patent No. 3271353) and a method in which a diol having basicity of the hydroxyl group equal to or higher than that of the dihydroxy compound used is added to the reaction system (patent document 12; JP Patent No. 3301453) have been proposed. However, a high molecular-weight polycarbonate resin having satisfactory physical properties cannot be obtained by any of these methods.
As apparent from the foregoing, the conventional methods for producing a high molecular-weight aromatic polycarbonate have many problems to be solved, and there are still demands for an improved method for producing a polycarbonate, which can achieve a polycarbonate having a satisfactorily increased molecular weight while maintaining excellent quality inherent in the polycarbonate.
The present inventors have previously found out a novel process, as a process for obtaining an aromatic polycarbonate which can accomplish a rapid polymerization rate and gives good quality, in which end-capped terminals of the aromatic polycarbonate is connected with an aliphatic diol compound to elongate the chain (Patent Document 13; WO 2011/062220A pamphlet). According to this process, end-capped terminals of the aromatic polycarbonates are linked to the aliphatic diol compound to elongate the chain, whereby an aromatic polycarbonate resin with a high polymerization degree having an Mw of about 30,000 to 100,000 can be produced within a short period of time. This method produces a polycarbonate by a polymerization reaction at a high rate, and therefore can suppress a branching or crosslinking reaction which is caused due to, for example, heat detention for a prolonged time, and can avoid the deterioration of, for example, hue of the resin.
Further, the present inventors have already proposed a method for producing a branched aromatic polycarbonate resin having a desired degree of branching, which comprises the step of subjecting an aromatic polycarbonate prepolymer having a branched structure introduced thereinto and an aliphatic diol compound to transesterification reaction in the presence of a transesterification catalyst under reduced pressure conditions (patent document 14; WO 2012/108510A pamphlet).
By the above methods for obtaining a polycarbonate having an increased molecular weight using a linking agent comprising an aliphatic diol compound, a polycarbonate resin having a satisfactorily increased molecular weight can be produced quickly with ease while maintaining excellent quality inherent in the polycarbonate. However, for producing a high molecular-weight polycarbonate resin having more excellent heat stability, a further improvement of these methods is desired.